Electrical power is generated, transmitted and distributed at various voltages, with transmission typically at hundreds of kV and distribution at 35 kV or less. System faults on alternating current networks are interrupted by equipment or devices which create an arc during the high current period of a half cycle and extinguish it at a natural current zero, which occurs twice each cycle. The difficulty and cost of interrupting a fault arc at current-zero increases with system voltage. Circuit breakers with separable contacts are typically used at transmission voltages and are highly complex and expensive equipment. At distribution voltages two fault interruption means are dominant. Reclosers (small circuit breakers) are used for transient faults and fuses, which must be manually replaced, are used for permanent faults requiring repair or replacement of other equipment. Expulsion fuses, which produce their own arc quenching gases, have historically been the dominant means for protection against permanent faults on overhead distribution systems.
Increasing demand for electrical power has resulted in distribution at higher voltages and has increased the required performance capabilities of distribution fuses in terms of voltage, load current, available fault current and transient recovery voltage frequency. Expulsion fuses are typically limited to 20 kA fault current interrupting ratings at a 5 kHz Transient Recovery Voltage frequency or lower. Current limiting fuses have been developed in the last decade or two for higher fault current duty (see U.S. Pat. No. 3,863,187 issued to William R. Mahieu, et al., which is herein incorporated by reference). However, load and voltage ratings are not always sufficient and their cost is high relative to distribution cutouts. There is an increasing need for distribution expulsion fuses with higher load and fault ratings at minimum costs.
Present-art expulsion fuses generate arc quench gases in a relatively long length (about 10 inches), large diameter bore (about 1 inch) arc tube and expel it to the ambient air where expansion as a free jet produces supersonic velocities over a short length. Thus, cooling of the decaying arc path between the two arc terminating electrodes in present-art expulsion fuses after current-zero is dependent upon two distinct processes: reduction in diameter of the subsonic plasma residual in said long and large arc bore and turbulent mixing of supersonic plasma flow external to the arc tube in the ambient air. These two processes are governed by different arc cooling time constants, with the subsonic cooling in the arc tube requiring more time and thereby limiting the fault interrupting performance against the rapidly rising system voltage across the two segments of the cooling arc path. If the system voltage increases too rapidly (transient recovery voltage frequency is too high) the arc will reheat and a thermal reignition of the arc will occur. Also, the peak value of the transient recovery voltage may exceed the dielectric strength of the deionized, but still hot decayed arc path and produce a dielectric reignition.
Companies which develop transmission circuit breakers have applied sophisticated experimental techniques and computer based numerical simulations to the current-zero arc interruption process in axial flow gas circuit breakers (see Klaus Ragaller, Editor, Current Interruption in High-Voltage Networks, Plenum Press, New York, 1978). Gas breakers have electrical contacts which separate along the axis of a single or dual Laval (converging, diverging) nozzle. They use stored high pressure gas for arc cooling at current-zero and complex timing and actuation means. That research has shown that nearly all of the network voltage develops across the supersonic, turbulent flow in the diverging nozzle section downstream from the throat or minimum flow area where sonic velocities are reached. Many of the breaker design parameters which significantly affect interruption performance have also been identified.
This invention was the result of an effort to apply recent knowledge of gas circuit breaker performance to a low cost, high performance expulsion fuse which generates its own arc quench gases for application on distribution power systems.
It is an object of the present invention to provide an improved expulsion fuse having increased supersonic flow characteristics.